Armature is deforming too much mesh on one side. Rotating hand also rotates foot
Sounds like Automatic Weight Paint failed.
Possible reason can be wrong Normal Face orientation.

In this example I also model separate parts of body. Later I joined right parts together. When I Mirror right side to left side in edit mode with Ctrl+M+X Normals switched ... hmm. Automatic Weight Paint issue followed.


To fix it check first normal face orientation, like enable Display Face Normals in edit mode. On left side is direction pointed inward.
(Or you can display Face Orientation in object mode, but since this view mode use red/blue colors same as weight paint, I didnt wanted to mess up screenshots with similar colors here.)


Recalculate Normals Shift+N

Go to object Data Properties &gt; Vertex Groups and Delete all Unlocked, and run Ctrl+P Parent &gt; Armature Deform &gt; With Automatic Weights again.

Note: I see red/blue color of face orientation vs WeightPaint Red/Blue is probably confusing here :)